Olefin streams containing light olefins, such as C3 to C5 olefins, may be used as feed for oligomerization units, forming dimers, trimers and other oligomers (low molecular weight polymers). The formation of these oligomers is conveniently accomplished using catalysts such as nickel or solid phosphoric acid (SPA). The product may be used as-is (e.g., as process fluid or gasoline blending stock) or converted to a variety of alkanes, esters, aldehydes, alcohols and acids for use as intermediates or end-products, such as detergents and plasticizers.
Two types of fixed bed reactors are typically employed with solid catalysts: chamber and tubular, generally described, for instance, in Hengstebeck, “Petroleum Processing”, McGraw-Hill (1959), pp. 208-234.
In a chamber reactor, two or more catalyst beds are incorporated into a single vessel in series. The temperature rise across each bed, due to the reaction, is cooled before the next bed with an external and cooler quench stream. This quench stream is generally predominantly saturated hydrocarbon material and often is the product distilled in the first fractionation tower downstream of the reactors in the process, or vapour condensate from a flash drum located in a similar position. Where a chamber reactor is used the heat produced by the exothermic olefin conversion reactions can be controlled by using a low reactivity hydrocarbon as a recycle from reactor effluent to reactor feedstock and/or as a quench between multiple catalyst beds within the reactor. The use of a chamber reactor for the oligomerization of light olefins to heavier olefins is described, for instance, in U.S. Pat. No. 6,072,093.
Tubular reactors typically are single-pass heat exchangers (e.g., shell-and-tube), with the catalyst normally contained in the tubes. The shell side typically contains a circulating heat exchange fluid. For reasons of more effective heat transfer, it is often preferred to select this fluid such that shell-side conditions can be applied under which the selected fluid at least partly evaporates. A convenient selection is in many instances to use water/steam because inter alia water is readily available, the temperature of the reactor can be controlled by controlling steam pressure, and the system is readily integrated with the water/steam systems typically present in many chemical and petrochemical operations. Water may be substituted by other fluids such as narrow boiling point hydrocarbons or hydrocarbon mixtures (e.g, narrow cut alkanes). The use of tubular reactors for the oligomerization of light olefins to heavier olefins is described, for instance, in U.S. Pat. No. 4,709,111.
See also, generally, WO 2005058777A1, for a discussion of tubular and chamber reactors in the oligomerization of light olefins and other hydrocarbon conversion processes that may be advantageously carried out therein using a variety of catalysts.
The use of molecular sieve catalysts in oligomerization of various feed streams to make olefins, including synergistic effects observed with a dual catalyst system, is known. See, for instance, U.S. Pat. Nos. 6,143,942; 6,770,791; and 6,875,899. However, although reactor design for chemical process is an area of active research and often focuses on approaching isothermal operation—see, for instance U.S. Patent Application Nos. 20030133858, 20040266893, and 20050061490—improvements are still needed in the design of reactors using more than one catalyst.
Other references of interest include U.S. Pat. Nos. 4,487,985; 4,547,609; 4,740,645; 4,919,896; 5,177,279; 5,177,282; and 5,672,800; GB Serial No. 0512377.3 (filed 17 Jun. 2005); and EMOGAS Technology for Catpoly Units, presented at the National Petrochemical & Refiners Association Annual Meeting, Mar. 13-15, 2005, San Francisco, Calif.
It has been observed by the present inventors that the use of zeolites in high throughput, one-pass tubular reactors results in deteriorated selectivity due at least in part to the inability, using current reactors, to control the temperature over the length of the catalyst bed. At least one underlying problem is that more than a desired portion of the total reaction is taking place in the earlier portion of the catalyst bed. One problem to be solved, then, is how to improve the distribution of the reaction over the total available reactor volume. It has further been observed by the present inventors that the performance of catalyst combinations is optimized when each catalyst can be run at a different optimized temperature. Another problem to be solved is to provide a low-cost reactor system providing such a capability.
Reactor design for isothermal operation has in the past focused mainly on tubular reactors and in particular to the temperature control system using their heat exchange fluid. One example of such design is shown in FIG. 1. The heat exchange fluid is assumed to be water. In this and other figures herein, because of the large number of valves they are not shown; however it is within the skill of the ordinary artisan to configure the appropriate valves. Water/steam exiting the reactor (not shown) enters via conduit 1 into (or near) the top of a conventional steam drum 10. The water in conduit 1 either drops into the steam drum 10 or is entrained in the steam produced by the heat of reaction and removed along with steam via conduit 2. The steam and entrained water that exits the steam drum 10 via conduit 2 is replaced by make-up water or boiler feed water (BFW) which is fed into the steam drum 10 via conduit 3, typically entering below the water level illustrated by line 11 in FIG. 1. The water entering conduit 3 typically is deaerated by bringing it close to or up to boiling temperature at about atmospheric pressure, and therefore this water is substantially colder than the water already in the drum, where the pressure is typically higher. Water exiting steam drum 10 via conduit 4 is returned to the reactor. As would be recognized by one of ordinary skill in the art, passage through steam drum 10 is induced by at least one of (i) forced flow, such as by mechanically pumping said water/steam, preferably the water in conduit 4, and (ii) thermosyphon circulation.
A reactor design providing for more isothermal operating conditions has recently been previously described in U.S. patent application Ser. No. 11/140,853, filed May 31, 2005. The invention described therein concerns a reactor system comprising at least one reactor and a drum and having a heat exchange fluid circulating through a first conduit from the reactor to the drum and a second conduit from the drum to the reactor, and wherein a portion of the fluid is removed from the reactor system and makeup fluid is added to the reactor system through a makeup fluid conduit connected to the drum, said drum having a liquid phase and a vapor phase above the liquid phase, the improvement comprising connecting the makeup fluid conduit and the first conduit into the same phase of the drum, e.g., both into the liquid phase or both into the vapor phase above the liquid phase. Embodiments are shown in FIGS. 2 through 4, wherein like numerals indicate like parts throughout the various embodiments.
FIG. 2 illustrates an embodiment of the invention described in the aforementioned application. As shown in FIG. 2, water/steam exiting the reactor (not shown) enters via conduit 1 into (or near) the top of steam drum 10. The water in conduit 1 either drops into the steam drum 10 or is entrained in the steam produced by the heat of reaction and removed along with steam via conduit 2. The steam and entrained water that exits the steam drum 10 via conduit 2 is replaced by make-up water or boiler feed water (BFW) which is fed into the steam drum 10 via conduit 3. According to the invention, the makeup water or BFW in conduit 3 is preheated by the water/steam in conduit 1 prior to entering steam drum 10 into the vapor phase above liquid level 11. In FIG. 2 water exiting steam drum 10 is returned to the reactor via conduit 4. No external source of heat is needed to preheat the BFW water (although that remains an optional choice). In another embodiment shown in FIG. 3, the improvement comprises having the water/steam in conduit 1 (exiting the reactor) entering the steam drum below the interface 11, the same phase for BFW entering from conduit 3. Elements 2, 4, and 10 serve the same function as the like numerals in FIGS. 1 and 2. In yet another embodiment shown in FIG. 4, the water/steam conduit 1 exiting the reactor enters the makeup water line 3 (below water/vapor interface 11) to preheat the makeup water prior to entering the drum 10. As in the other embodiments, water returns to the reactor via conduit 4 and steam exits via conduit 2.
However, in such a reactor system, while two different catalysts may be used, e.g, by stacking one atop the other or by mixing the two catalysts, the temperature selected is necessarily a compromise somewhere between the optimal operating conditions of each individual catalyst.
Solid Phosphoric Acid (SPA) catalyst is particularly useful for producing C13 and lower oligomers, and is particularly highly selective for nonene from propylene, which is highly valued in the Oxo Process to produce plasticizer alcohols (with regard to the Oxo Process, see for example WO2005058787 and WO2003082789A2, which in turn recite numerous references to the same subject matter).
However, SPA catalyst deactivates quickly relative to zeolite catalysts, and fails more quickly at higher temperatures. SPA catalyst swells and agglomerates with time on stream causing a steady erosion of porosity, and must be removed after service by drilling or water jetting. Because SPA catalyst runs are usually terminated due to excessive pressure drop, running SPA catalyst in series reactors is less practical. As far as the present inventors are aware, a previously unrecognized benefit of zeolite catalyst is that zeolites do not build pressure drop with time on stream; this property of zeolites makes the use of zeolites in at least one reactor in a series of reactors particularly attractive.
Unlike solid phosphoric acid, zeolite catalysts typically have a higher selectivity to the desired gasoline boiling range product when operated at a higher temperature (typically 240-260° C.) and also typically produce large amounts of product boiling in the distillate carbon number range when operated at high light olefin conversion at the constant higher temperature (typically 240-260° C.). For this reason (among others), the economics of running a light olefin oligomerization plant with zeolite catalyst are improved by providing the unit operators with the ability to maintain a constant zeolite catalyst activity. Constant zeolite activity can be approached by allowing the temperature of the unit to vary with time.
Thus, each catalyst has it benefits, but clearly a compromise temperature is not the ideal solution when running a dual catalyst system using SPA and zeolites or even two different zeolites.
It would be beneficial if the positive attributes of each catalyst for oligomerization could be combined without the attendant drawbacks. There is a need, particularly, for a method to enable reactor severity control in light olefin polymerization units, to maintain isothermality, and to allow, in the case of a dual catalyst system, each catalyst to operate at optimal activity and selectivity vis-a-vis the desired outcome (e.g., a uniform product or more of the desired products).
The present inventors have surprisingly discovered that a reactor system comprising at least two catalysts beds in series, each catalyst bed having a different catalyst and each catalyst bed provided with independent temperature control, such as with independent steam drums in the case of tubular reactors or separate quench as in the case of chamber reactors, provides for improved performance in oligomerization reactions.